Integrated micro-optical systems

ABSTRACT

An integrated micro-optical system includes at least two wafers with at least two optical elements provided on respective surfaces of the at least two wafers. An active element having a characteristic which changes in response to an applied field may be integrated on a bottom surface of the wafers. The resulting optical system may present a high numerical aperture. Preferably, one of the optical elements is a refractive element formed in a material having a high index of refraction.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation and claims priority under 35U.S.C. §120 to U.S. application Ser. No. 09/961,303, filed Sep. 25,2001, to be issued as U.S. Pat. No. 6,483,627 on Nov. 19, 2002, whichclaims priority under35 U.S.C. §120 to U.S. application Ser. No.09/566,818, filed May 8, 2000, now U.S. Pat. No. 6,295,156 issued onSep. 25, 2001, which claims priority under 35 U.S.C. §120 to U.S.application Ser. No. 09/276,805, filed on Mar. 26, 1999, now U.S. Pat.No. 6,061,169 issued on May 9, 2000, which claims priority under 35U.S.C. §119 to U.S. Provisional Application No. 60/079,378 filed on Mar.26, 1998, the entire contents of all of which are hereby incorporated byreference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to integrating optics on the waferlevel with an active element, particularly for use with magneto-opticheads.

2. Description of Related Art

Magneto-optical heads are used to read current high-densitymagneto-optic media. In particular, a magnetic coil is used to apply amagnetic field to the media and light is then also delivered to themedia to write to the media. The light is also used to read from themedia in accordance with the altered characteristics of the media fromthe application of the magnetic field and light.

An example of such a configuration is shown in FIG. 1. In FIG. 1, anoptical fiber 8 delivers light to the head. The head includes a sliderblock 10 which as an objective lens 12 mounted on a side thereof. Amirror 9, also mounted on the side of the slider block 10, directs lightfrom the optical fiber 8 onto the objective lens 12. A magnetic coil 14,aligned with the lens 12, is also mounted on the side of the sliderblock 10. The head sits on top of an air bearing sandwich 16 which isbetween the head and the media 18. The slider block 10 allows the headto slide across the media 18 and read from or write to the media 18.

The height of the slider block 10 is limited, typically to between500-1500 microns, and is desirably as small as possible. Therefore, thenumber of lenses which could be mounted on the slider block is alsolimited. Additionally, alignment of more than one lens on the sliderblock is difficult. Further, due to the small spot required, the opticsor overall optical system of the head need to have a high numericalaperture, preferably greater than 0.6. This is difficult to achieve in asingle objective lens due to the large sag associated therewith. Theoverall head is thus difficult to assemble and not readily suited tomass production.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a sliderblock having an active element, i.e., an element having a characteristicwhich changes in response to an applied field, integrated thereon whichsubstantially overcomes one or more of the problems due to thelimitations and disadvantages of the related art. Such elements includea magnetic coil, a light source, a detector, etc.

It is a further object of the present invention to integrate multipleoptical elements and a slider block having the active element integratedthereon as well. It is a further object of the present invention tomanufacture the objects on a wafer level, bond a plurality of waferstogether and provide the active element on a bottom surface of a bottomwafer.

At least one of the above and other advantages may be realized byproviding an integrated micro-optical system including a die formed frommore than one wafer bonded together, each wafer having a top surface anda bottom surface, bonded wafers being diced to yield multiple dies andan active element having a characteristic which changes in response toan applied field, integrated on a bottom surface of the die, opticalelements being formed on more than one surface of the die.

The active element may be a thin film conductor whose magneticproperties change when a current is applied thereto. The active elementmay be integrated as an array of active elements on the bottom waferbefore the bonded wafers are diced. The die may be formed from twowafers and optical elements are formed on a top surface and a bottomsurface of a top wafer and a top surface of the bottom wafer. The diemay include a high numerical aperture optical system.

The bottom wafer of the more than one wafer may have a higher index ofrefraction than other wafers. There may be no optical elements on abottom wafer of the die. The bottom surface of the die may furtherinclude features for facilitating sliding of the integratedmicro-optical system etched thereon. The bottom wafer of the die mayhave a refractive element formed in a material of high numericalaperture. Metal portions serving as apertures may be integrated on atleast on one of the surfaces of the die.

A layer of material deposited on the bottom surface of the bottom waferbefore the active element is integrated thereon. An optical element maybe formed on the bottom surface of the bottom wafer, wherein the layerhas a refractive index that is different from the refractive index ofthe bottom wafer. The layer may be deposited in accordance with adifference between a desired thickness and a measured thickness.

A monitoring optical system may be formed on each surface of the wafercontaining an optical element. The spacing between wafers may be variedin accordance with a difference between a measured thickness of a waferand a desired thickness of a wafer.

A top surface of the die may be etched and coated with a reflectivecoating to direct light onto the optical elements. A further substratemay be mounted on top of the top of the die having a MEMS mirrortherein. An insertion point may be provided on the die for receiving anoptical fiber therein. The insertion point may be on a side of the dieand the system further includes a reflector for redirecting light outputby the fiber.

A refractive element in the die may be a spherical lens and the diefurther includes a compensating element which compensates foraberrations exhibited by the spherical lens. The compensating elementmay be on a surface immediately adjacent the spherical lens. Thecompensating element may be a diffractive element. The refractiveelement may be an aspheric lens. The die may include at least oneadditional refractive element, all refractive elements of the die beingformed in material having a high numerical aperture.

At least one of the above and other advantages may be realized byproviding an integrated micro-optical apparatus including a die formedfrom more than one wafer bonded together, each wafer having a topsurface and a bottom surface, bonded wafers being diced to yieldmultiple die, at least two optical elements being formed on respectivesurfaces of each die, at least one of the at least two optical elementsbeing a refractive element, a resulting optical system of each diehaving a high numerical aperture.

The refractive element may be a spherical lens and the die furtherincludes a compensating element which compensates for aberrationsexhibited by the spherical lens. The compensating element may be on asurface immediately adjacent the spherical lens. The compensatingelement may be a diffractive element. The refractive element may be anaspheric lens.

The die may include at least one additional refractive element, allrefractive elements of the die being formed in material having a highnumerical aperture. The refractive element may be on a bottom wafer andof a material having a higher refractive index than that of the bottomwafer.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 illustrates a configuration of a high-density flying headmagneto-optical read/write device;

FIG. 2A illustrates one configuration for the optics to be used informing a slider block;

FIG. 2B illustrates the spread function of the optical system shown inFIG. 2A;

FIG. 3A illustrates a second embodiment of the optics for use in slidingblock of the present invention;

FIG. 3B illustrates the spread function of the optical system shown inFIG. 3A;

FIG. 4A illustrates a third embodiment of an optical system to be usedin the slider block of the present invention;

FIG. 4B illustrates the spread function of the optical system shown inFIG. 4A;

FIG. 5 is a side view of an embodiment of a slider block in accordancewith the present invention;

FIG. 6 is a side view of another embodiment of a slider block inaccordance with the present invention;

FIG. 7 is a side view of another embodiment of a slider block inaccordance with the present invention;

FIG. 8A is a side view of another embodiment of a slider block inaccordance with the present invention; and

FIG. 8B is a bottom view of the embodiment in FIG. 8A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

All of the optical systems shown in FIGS. 2A-4B provide satisfactoryresults, i.e., a high numerical aperture with good optical performance.The key element in these optical systems is the distribution of theoptical power over multiple available surfaces. Preferably thisdistribution is even over the multiple surfaces. Sufficient distributionfor the high numerical aperture required is realized over more than onesurface. Due to the high numerical aperture required, this distributionof optical power reduces the aberrations from the refractive surfacesand increases the diffractive efficiency of the diffractive surfaces byreducing the deflection angle required from each surface.

Further, a single refractive surface having a high numerical aperturewould be difficult to incorporate on a wafer, since the increasedcurvature required for affecting such a refractive surface would resultin very thin portions of a typical wafer, leading to concerns aboutfragility, or would require a thick wafer, which is not desirable inmany applications where size is a major constraint. Further, the preciseshape control required in the manufacture of a single refractive surfacehaving high NA would present a significant challenge. Finally, thesurfaces having the optical power distributed are easier to manufacture,have better reproducibility, and maintain a better quality wavefront.

In accordance with the present invention, more than one surface may beintegrated with an active element such as a magnetic coil by bondingwafers together. Each wafer surface can have optics integrated thereonphotolithographically, either directly or through molding or embossing.Each wafer contains an array of the same optical elements. When morethan two surfaces are desired, wafers are bonded together. When thewafers are diced into individually apparatuses, the resulting product iscalled a die. The side views of FIGS. 2A, 3A, and 4A illustrate suchdies which consist of two or three chips bonded together by a bondingmaterial 25.

In the example shown in FIG. 2A, a diffractive surface 20 is followed bya refractive surface 22, which is followed by a diffractive surface 24,and then finally a refractive surface 26. In the example shown in FIG.3A, a refractive surface 30 is followed by a diffractive surface 32,which is followed by a refractive surface 34 which is finally followed adiffractive surface 36. In the optical system shown in FIG. 4A, arefractive surface 40 is followed by a diffractive surface 42 which isfollowed by a refractive surface 44 which is followed by a diffractivesurface 46, which is followed by a refractive surface 48 and finally adiffractive surface 50. The corresponding performance of each of thesedesigns is shown in the corresponding intensity spread function of FIGS.2B, 3B, and 4B.

When using spherical refractive elements as shown in FIGS. 2A, 3A and4A, it is convenient to follow these spherical refractive elements witha closely spaced diffractive element to compensate for the attendantspherical aberration. An aspherical refractive does not exhibit suchaberrations, so the alternating arrangement of refractives anddiffractives will not necessarily be the preferred one.

While the optical elements may be formed using any technique, to achievethe required high numerical aperture, it is preferable that therefractive lenses remain in photoresist, rather than being transferredto the substrate. It is also preferable that the bottom substrate, i.e.,the substrate closest to the media, has a high index of refractionrelative of fused silica, for which n=1.36. Preferably, this index is atleast 0.3 greater than that of the substrate. One example candidatematerial, SF56A, has a refractive index of 1.785. If the bottomsubstrate is in very close proximity to the media, e.g., less than 0.5microns, the use of a high index substrate allows a smaller spot size tobe realized. The numerical aperture N.A. is defined by the following:

N.A.=n sin θ

where n is the refractive index of the image space and θ is thehalf-angle of the maximum cone of light accepted by the lens. Thus, ifthe bottom substrate is in very close proximity to the media, the higherthe index of refraction of the bottom substrate, the smaller theacceptance half-angle for the same performance. This reduction in angleincreases the efficiency of the system.

As shown in FIG. 5, the slider block 61 in accordance with the presentinvention includes a die composed of a plurality of chips, each surfaceof which is available for imparting optical structures thereon. The dieis formed from wafers having an array of respective optical elementsformed thereon on either one or both surfaces thereof. The individualoptical elements may be either diffractive, refractive or a hybridthereof. Bonding material 25 is placed at strategic locations on eithersubstrate in order to facilitate the attachment thereof. By surroundingthe optical elements which are to form the final integrated die, thebonding material or adhesive 25 forms a seal between the wafers at thesecritical junctions. During dicing, the seal prevents dicing slurry fromentering between the elements, which would result in contaminationthereof. Since the elements remain bonded together, it is nearlyimpossible to remove any dicing slurry trapped there between. The dicingslurry presents even more problems when diffractive elements are beingbonded, since the structures of diffractive elements tend to trap theslurry.

Advantageously, the wafers being bonded include fiducial marks somewherethereon, most likely at an outer edge thereof, to ensure alignment ofthe wafers so that all the individual elements thereon are alignedsimultaneously. Alternatively, the fiducial marks may be used tofacilitate the alignment and creation of mechanical alignment featureson the wafers. One or both of the fiducial marks and the alignmentfeatures may be used to align the wafers. The fiducial marks and/oralignment features are also useful in registering and placing the activeelements and any attendant structure, e.g., a metallic coil and contactpads therefor, on a bottom surface. These active elements could beintegrated either before or after dicing the wafers.

On a bottom surface 67 of the slider block 61 in accordance with thepresent invention, a magnetic head 63 including thin film conductorsand/or a magnetic coil is integrated using thin film techniques, asdisclosed, for example, in U.S. Pat. No. 5,314,596 to Shukovsky et al.Entitled “Process for Fabricating Magnetic Film Recording Head for usewith a Magnetic Recording Media.” The required contact pads for themagnetic coil are also preferably provided on this bottom surface.

Since the magnetic coil 63 is integrated on the bottom surface 67, andthe light beam is to pass through the center of the magnetic coil, it istypically not practical to also provide optical structures on thisbottom surface. This leaves the remaining five surfaces 50-58 availablefor modification in designing an optical system. Further, additionalwafers also may be provided thereby providing a total of seven surfaces.With the examples shown in FIGS. 2A and 3A the surface 50 wouldcorrespond to surface 20 or 40, respectively, the surface 52 wouldcorrespond to surface 22 or 32, respectively, the surface 54 wouldcorrespond to surface 24 or 34, respectively, and the surface 56 wouldcorrespond to surface 26 or 36, respectively.

Each of these wafer levels can be made very thin, for example, on theorder of 125 microns, so up to four wafers could be used even under themost constrained conditions. If size and heat limitations permit, alight source could be integrated on the top of the slider block, ratherthan using the fiber for delivery of light thereto. In addition to beingthin, the use of the wafer scale assembly allows accurate alignment ofnumerous objects, thereby increasing the number of surfaces containingoptical power, which can be used. This wafer scale assembly also allowsuse of passive alignment techniques. The other dimensions of the sliderblock 61 are determined by the size of the pads for the magnetic coil,which is typically 1500 microns, on the surface 67, which is going to bemuch larger than any of the optics on the remaining surfaces, and anysize needed for stability of the slider block 61. The bottom surface 67may also include etch features thereon which facilitate the sliding ofthe slider block 61.

Many configurations of optical surfaces may be incorporated into theslider block 61. The bonding, processing, and passive alignment ofwafers is disclosed in co-pending, commonly assigned U.S. applicationSer. No. 08/727,837 entitled “Integrated Optical Head for Disk Drivesand Method of Forming Same” and U.S. application Ser. No. 08/943,274entitled “Wafer Level Integration of Multiple Optical Heads” which areboth hereby incorporated by reference in their entirety.

Additionally, an optical element can be provided on the bottom surface67 of the bottom wafer as shown in FIG. 6. When providing an opticalelement on this bottom surface 67, a transparent layer 65, having adifferent refractive index than that of the wafer itself is providedbetween the bottom surface 67 and the coil 63. The difference inrefractive index between the layer 65 and the wafer should be at leastapproximately 0.3 in order to insure that the optical effect of theoptical element provided on the bottom surface 67 is discernable. Alsoas shown in FIG. 6, a single wafer may be used if sufficient performancecan be obtained from one or two optical elements.

Further as shown in FIG. 6, metal portions 69 may be provided to serveas an aperture for the system. These apertures may be integrated on anyof the wafer surfaces. The aperture may also serve as the aperture stop,typically somewhere in the optical system prior to the bottom surfacethereof. When such metal portions 69 serving as an aperture are providedon the bottom surface 67, it is advantageous to insure the metalportions 69 do not interfere with the operation of the metal coil 63.

A problem that arises when using a system with a high numerical aperturefor a very precise application is that the depth of focus of the systemis very small. Therefore, the distance from the optical system to themedia must be very precisely controlled to insure that the beam isfocused at the appropriate position of the media. For the high numericalapertures noted above, the depth of focus is approximately 1 micron orless. The thicknesses of the wafers can be controlled to withinapproximately 1-5 microns, depending on the thickness and diameter ofthe wafer. The thinner and smaller the wafer, the better the control.When multiple wafers are used, the system is less sensitive to avariation from a design thickness for a particular wafer, since thepower is distributed through all the elements.

When using multiple wafers, the actual thickness of each wafer can bemeasured and the spacing between the wafers can be adjusted to accountfor any deviation. The position of the fiber or source location can beadjusted to correct for thickness variations within the wafer assembly.Alternatively, the design of a diffractive element may be altered inaccordance with a measured thickness of the slider block in order tocompensate for a variation from the desired thickness. Alternatively,the entire system may be designed to focus the light at a positiondeeper than the desired position assuming the thicknesses are preciselyrealized. Then, the layer 65 may be deposited to provide the remainingrequired thickness to deliver the spot at the desired position. Thedeposition of the layer 65 may be more precisely controlled than theformation of the wafers, and may be varied to account for any thicknessvariation within the system itself, i.e., the layer 65 does not have tobe of uniform thickness. If no optical element is provided on the bottomsurface 67, then the refractive index of the layer 65 does not need tobe different from that of the wafer.

FIG. 7 is a side view of another embodiment of the slider block. Asshown in FIG. 7, the fiber 8 is inserted into the top wafer and themirror 9 is integrated into the top wafer, preferably at a 45-degreeangle. Light reflected by the mirror 9 is directed to a diffractiveelement 71, followed by a refractive element 73, followed by adiffractive element 75, followed by a refractive element 77, anddelivered through the coil 63. For such a configuration, the top surface50 is no longer available for providing an optical element.

Additionally, for fine scanning control of the light, the mirror 9 maybe replaced with a micro-electro-mechanical system (MEMS) mirror mountedon a substrate on top of the top chip. A tilt angle of the MEMS iscontrolled by application of a voltage on a surface on which thereflector is mounted, and is varied in accordance with the desiredscanning. The default position will preferably be 45 degrees so theconfiguration will be the same as providing the mirror 9.

An additional feature for monitoring the spot of light output from theslider block is shown in FIGS. 8A and 8B. As shown in FIG. 8A, inaddition to the optical system, consisting of, for example, diffractiveelements 87, 89, used for delivering light through the magnetic coil 63,monitoring optical elements 81, 83 are provided. The monitoring opticalelements 81, 83 are of the same design as the elements of the opticalsystem 87, 89, respectively. In other words, the monitoring opticalelements are designed to focus at a same distance as that of the opticalsystem. Advantageously, the monitoring optical elements 81, 83 arelarger than the optical system elements for ease of construction andalignment of the test beam. In the configuration shown in FIGS. 8A and8B, the monitoring optical elements 81, 83 are approximately twice thesize of the element 87, 89. The monitoring system also includes anaperture 85, preferably formed by metal. It is noted that FIG. 8B doesnot show the magnetic coil 63.

During testing, light is directed to the monitoring optical system toinsure that light is being delivered to the aperture at the desiredlocation. The magnitude of light passing through the aperture willindicate if the optical system is sufficiently accurate, i.e., that thelight is sufficiently focused at the aperture to allow a predeterminedamount of light through. If the light is not sufficiently focused, theaperture will block too much of the light.

Thus, by using the monitoring system shown in FIGS. 8A and 8B, theoptical system of the slider block may be tested prior to its insertioninto the remaining device, even after being integrated with the activeelement 63. The dimension requirement imposed by the contact pads forthe magnetic coil 63 and the coil itself result in sufficient roomavailable on the wafers for the inclusion of such a monitoring system,so the size of the slider block is unaffected by the incorporation ofthe monitoring system.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed:
 1. An integrated micro-optical system comprising: afirst wafer having a first top surface and a first bottom surface; asecond wafer having a second top surface and a second bottom surface; afirst optical element on one of the first top surface, the first bottomsurface, the second top surface and the second bottom surface; and anactive element in communication with the second bottom surface.
 2. Thesystem of claim 1, wherein the active element is on the second bottomsurface.
 3. The system of claim 1, wherein the first and second wafersare secured together.
 4. The system of claim 1, further comprising asecond optical element on one of the first top surface, the first bottomsurface, the second top surface and the second bottom surfaces thesecond optical element being on a different surface than the firstoptical element.
 5. The system of claim 4, wherein the first opticalelement and the second optical element are on different wafers.
 6. Thesystem of claim 1, wherein metal portions serving as apertures areintegrated on at least one of the first top surface, the first bottomsurface, the second top surface and the second bottom surface.
 7. Thesystem of claim 1, further comprising means for varying a spacingbetween wafers in accordance with a difference between a measuredthickness of a wafer and a desired thickness of a wafer.
 8. The systemof claim 1, wherein the first top surface of is etched and coated with areflective coating to direct light onto the optical elements.
 9. Thesystem of claim 1, further comprising a third substrate mounted on topof the first top surface, the third substrate having a MEMS mirrorthereon.
 10. The system of claim 1, further comprising an opticalwaveguide, the first optical element coupling tight between the activeelement and the optical waveguide.
 11. The system of claim 1, wherein,the first and second wafers are secured together on a wafer level. 12.The system of claim 1, wherein the first optical element is a refractiveelement.
 13. The system of claim 12, wherein the refractive element is aspherical lens.
 14. The system of claim 1, further comprising a monitorsystem on the first and second wafers.